1. Field of the Invention
This invention relates generally to an instrument that provides the magnetic direction of a vehicle to which it is mounted for navigation purposes, and more specifically to an Electronic Digital Compass system that senses, computes, and displays the vehicle's magnetic direction digitally.
2. Background of the Invention
The most fundamental vehicle navigation requirement is the direction in which the vehicle is traveling, and for about the last 2000 years the most fundamental navigation instrument has been the magnetic compass. For the great majority of its existence the compass has been a mechanical instrument that has slowly evolved over its great life span. The advent of an electronic compass did not occur until the 20th century, with its great electronic revolution.
The electronic compass is a single example of a general class of instruments called magnetometers, and more specifically it is classified as a vector magnetometer. This is because the earth's magnetic field is a 3 dimensional vector and the purpose of the compass is to determine the vehicles direction (magnetic heading) relative to the two horizontal components of this vector field.
While there have been many forms of magnetometers developed, most electronic compass systems use what is called a flux-gate magnetometer. The flux-gate vector magnetometer is based upon a high permeability core material where the earth's field, external to the core and along its axis, is alternately pulled into and released from the core material. This is what is referred to as gating the external field.
A complete analysis of the flux-gate mechanism has been reported by a number of authors, for example: D. I. Gordon, et al., IEEE Transactions On Magnetics, MAG 1, No. 4, Dec. 1965, "Factors Affecting the Sensitivity of Gama-Level Ring Core Magnetometers"; S. V. Marshall, IEEE Transactions On Magnetics, MAG 3, Sept, 1967, "An Analytic Model For the Flux-Gate Magnetometer"; F. Primdahl, IEEE Transactions On Magnetics, MAG 6, No. 2, June 1970, "The Gating Curves of Parallel and Orthogonal Flux-Gates"; J. R. Burger, IEEE Transactions On Magnetics, MAG 8, No. 4, Dec. 1972, "The Theoretical Output of a Ring Core Flux-Gate Magnetometer"; and D. I. Gordon et al., IEEE Transactions on Magnetics, MAG 8, No. 1, Mar. 1972, "Recent Advances in Flux-Gate Magnetometry". The basics of the flux-gate theory will be explained by the following.
FIG. 1 shows the general shape of a B-H curve 10 of a typical soft magnetic material that would be used as the core material in flux-gate magnetometers. Mu-metal, permalloy, or super-mallow are examples of these high permeability materials. The B-H curve 10 shows the relationship of the magnetic field (H) on the horizontal axis applied to the core versus the magnetic flux (B) induced in the core on the vertical axis. When used in a flux gate magnetometer the permeability state of the core is what causes the gating action. The permeability is related to B and H by:
permeability=u=B/H PA1 Vs1=Amplitude of induced sense pulses. PA1 E=Magnitude of earth's field vector E PA1 .theta.=Angle of core relative to earth's field vector E. PA1 Vs1=Second harmonic sense signal from core 1. PA1 Vs2=Second harmonic sense signal from core 2. PA1 E=Magnitude of the earth's field. PA1 .theta.=Heading of the vehicle. PA1 w=Two Pi times the drive frequency. PA1 A=E*cos.theta. PA1 B=E*sin.theta. PA1 A=E*cos.theta. PA1 B=E*sin.theta. PA1 C=SQRT(A.sup.2 +B.sup.2) PA1 C=SQRT(E.sup.2 cos.sup.2 .theta.+E.sup.2 sin.sup.2 .theta.) PA1 C=E*SQRT(cos.sup.2 .theta.+sin.sup.2 .theta.) PA1 C=E*SQRT( 1 )=E PA1 D=.theta.
As can be seen from the B-H curve 10, at high values of H the flux density B stays approximately constant. That is, the core is said to be saturated, and in this saturated state the permeability of the core is small. In a second state the core is not saturated, which results in the value of the permeability being high.
Turning now to FIG. 2, in the high permeability, unsaturated state, the core material 12 acts as a low impedance path to the earth's magnetic flux lines 14 that lie along the core's axis, and the flux 14 is drawn into the core 12. In the other state of the core 12, when it is saturated and therefore of low permeability, the core 12 no longer presents a low impedance path to the flux 14 so it returns to the area outside of the core. This is shown in FIG. 3. These two distinct states of the core, the pulling, in of the earth's magnetic flux, and letting it return to its original position is the gating of the flux of the earth's field.
Both FIGS. 2 and 3 show a sense coil 16 wound around the core 12. When the core 12 changes from the saturated state to the unsaturated state and the earth's magnetic field flux is pulled into the core, the flux lines 14 "cut" (pass through) the turns of the sense coil 16 and induce a voltage pulse in the coil 16. When the core 12 then passes into the saturated state and the flux 14 leaves the core 12 a voltage pulse of equal amplitude, but of opposite polarity is induced. The amplitude of these pulses are directly proportional to the magnitude of that vector E the earth's magnetic field which lies parallel to the axis of the sense coil and core. It should also be noted that the polarity of the pulses depends upon the direction of the earth's field vector E with respect to the sense coil 16 and core 12.
Turning now to FIG. 4, as stated before, the gating of the flux is accomplished by having two distinct states of the core material. In order to control the state of the core a second coil 18, which is called the drive coil 18 is added around the core 12. This drive coil 18 is driven with an alternating current of sufficient amplitude to drive the core 12 from the saturated state through the unsaturated state and into the saturated state in the opposite direction.
FIG. 5 shows the relationship of the alternating drive current in the drive coil 18 to the sense voltage pulses produced in the sense coil 16 as the core 12 is rotated relative to the earth's magnetic field vector E. The amplitude of the induced sense coil voltage pulses is proportional to: EQU Vs1.varies.E*cos(.theta.)
where:
From this equation it can be seen that the same amplitude and polarity of the pulses will occur at two separate angles (.theta.) as the core is rotated through 360 degrees. To resolve this ambiguity a second core, with sense and drive coils, is placed at a 90 degree angle with the first core. This 90 degree physical space relationship of the second core relative to the first means that its induced sense coil voltage will be proportional to: EQU Vs2.varies.E*sin(.theta.)
and since: ##EQU1##
Then the heading of a vehicle, to which these orthogonal cores would be mounted as an electronic compass, can be calculated with: EQU .theta.=ArcTan(Vs2/Vs1)
Again from this equation it is seen that an electronic compass based upon a flux-gate magnetometer utilized the amplitude and phases of the voltages induced into the sense windings to calculate the heading of the vehicle.
When FIG. 5 is studied it is evident that each sense coil produces two pulses for each cycle of the alternation drive current. Thus the output of the sense coil is a waveform containing a large component of the second harmonic of the drive current. A magnetometer operating in this mode is called a second harmonic flux-gate magnetometer and is by far the most popular version in use at this time.
It can also be seen, when looking at FIG. 4, that the alternating current applied to the drive winding 18 will be induced into the sense winding 16, inundating the small pulses produced by gating the external field. Over the years there have been various strategies of core, sense coil, and drive coil configurations for saturating the core without inducing the drive signal into the sense winding. These strategies fall into two categories, parallel and orthogonal core configurations.
In the parallel configurations as shown in FIG. 6, the drive current is applied in opposite directions to two parallel cores 20 and 22, so that the resulting field seen by the sense winding(s) is effectively canceled. With experience these separate parallel cores have evolved into a toroidal core configuration as shown in FIG. 7 which again is the most popular version in use at this time.
In another variation shown in FIG. 8, known as orthogonal configurations, the saturating field generated by the alternating drive current is directed at right angles to the sense coil and so cannot induce a voltage into it. In the embodiment on the left, a tubular core is wound with a drive coil 18. In the embodiment on the right, a ferromagnetic wire 24 is driven by drive current applied to terminals 26.
As was stated before, a magnetometer operating as described is called a second harmonic flux-gate magnetometer. The performance at this second harmonic frequency is usually enhanced by capacitively tuning the sense coil to this frequency. This tuning also further discriminates against components of the drive signal which might still be induced into the sense coil.
While tuning of the sense coil can enhance the second harmonic signal it can also affect the accuracy of the compass system. Unless both sense coils are accurately tuned to exactly the second harmonic of the drive frequency, amplitude variations and large phase shifts of the second harmonic signals will be generated between the two axis outputs. Since the calculated heading is proportional to the ratio of the two amplitudes, any variation in one signal relative to the other will be seen as an error in the calculated heading.
When a second harmonic flux-gate magnetometer is being used as the sensor for an electronic digital compass, the circuitry required to produce a digital reading of the magnetic heading of a vehicle from the output of the magnetometer fall into two categories. The first is called amplitude conversion, and the second is called phase conversion.
A block diagram of a general amplitude conversion scheme is shown in FIG. 9. A square wave drive signal generated by a signal generator 30 is applied to the drive coil 32 which is coupled to the core 34. Two tuned second harmonic signals from orthogonal sense windings 36 and 38, tuned respectively by tuning capacitors 40 and 42, are directed to two parallel amplitude signal processing paths. The first block of signal processing in each path is a switching demodulator 46 and 48 that is driven by two times the core drive frequency from frequency multiplier 50. These demodulator blocks effectively do a full wave rectification of the second harmonic output of the sense windings, while preserving the amplitude information of the signal. It is at this point where large errors in the heading reading can be introduced if both sense windings 36 and 38 are not exactly tuned to the second harmonic of the drive frequency. Large phase shifts result from small differences between the resonant frequency of the tuned sense coils and frequency of the second harmonic drive signal. The demodulators 46 and 48 convert these phase variations into amplitude variations, thereby modifying the information carrying variable, signal amplitude.
The second block of each path is usually a low pass filter and a gain stage 54 and 56 respectively. The low pass filter turns the full wave rectified signal into a DC signal, and the gain amplifies it to a level required for further signal conversion. This DC gain is usually variable, so that differential amplitude errors between the two separate paths can be adjusted out.
The next blocks in each path are analog to digital converters 58 and 60 respectively which take the DC voltages that are proportional to either the sine or cosine of .theta. and convert them to two digital words.
These two digital words are then directed to a microprocessor 62 where the following calculation is performed: EQU .theta.=ArcTan(sin.theta./cos.theta.)
Usually the microprocessor 62 also does software filtering, data formatting, etc. and transmits the digital heading reading to a display 64.
A block diagram of a phase conversion scheme is shown in FIG. 10. A general description of the theory of this conversion technique can best be explained with equations.
The output signals from the two tuned sense windings 36 and 38 are proportional to: EQU Vs1.varies.E*cos.theta.* sin(2wt)=A*sin(2wt) EQU Vs2.varies.E*sin.theta.* sin(2wt)=B*sin(2wt)
where:
and:
The signal from the second sense winding 36 is then phase shifted by 90 degrees at twice the drive frequency at phase shifter 70. That is: EQU Vs2.sub.90 .varies.B*sin(2wt+90)=B*cos(2wt).
The 90 degree phase shifted output of sense coil 36 is then added to the output of coil 38 at summer 72. This sum becomes: EQU Vt=Vs1+Vs2.sub.90 EQU Vt=(A*sin(2wt))+(B*cos(2wt))
which by phaser addition becomes: EQU Vt=C*sin(2wt+D)
where: EQU C=SQRT (A.sup.2 +B.sup.2) EQU sin(D)=B/SQRT(A.sup.2 +B.sup.2)
with SQRT being the square root function. Knowing that:
then:
and: EQU sin D=B/SQRT(A.sup.2 +B.sup.2)=B/C EQU sin D=E*sin.theta./E=sin.theta.
therefore:
and: EQU Vt =E*sin(2wt+.theta.) .
From this equation it can be seen that the heading information contained in the amplitudes of the two sense signals is converted to a phase shift between the converted signal and two times the drive signal. This phase shift is detected by detector 74 and the phase shift is converted to a time difference pulse at the output of detector 74. This output is timed by a counter 76 which is driven by a crystal 78 controlled clock. The counter output is processed by microprocessor 62 which drives display 64. While this scheme combines the information wanted, that it heading .theta., early in the signal processing scheme, it also is vulnerable to tuning causing errors in the heading reading it produces.
The large phase shifts that result from small differences between the resonant frequency of the tuned sense coils and the second harmonic of the drive signal show up as errors in .theta.. Also the 90 degree phase shift circuit must accurately give 90 degrees, even if the drive frequency changes slightly, or more errors will be introduced. And finally, the gain of the two paths up to and through the point of summing must be balanced, or still more errors will be added to the heading reading.
After the summing point in the phase conversion scheme, the combined signal's phase shift relative to the second harmonic of the drive signal is converted to a time difference and used to count a reference clock with a counter 76 as described above. The number in the counter is now proportional to .theta. and is then directed to a microprocessor 62. The microprocessor 62 again does software filtering, data formatting, and sends the digital reading of heading to the display 64.
From the preceding discussion it can be seen that up to now electronic digital compass systems have really been an extension of analog flux-gate systems that have been in use since before the second world war. In general it can be said that they are analog sensors to which an analog to digital conversion is applied to obtain a digital output. The largely analog processing schemes are prone to introduction of errors from many sources, and require substantial tuning for accuracy. The conversion schemes may vary as described above, but the heading information is still contained as an analog amplitude output from the sensor.
In U.S. Pat. No. 4,591,788 to Mohri et al, a sensor for magnetic field sensing uses a twisted ribbon of amorphous magnetic material to produce a pulse in response to the presence of a external magnetic field similar to the so called "Wiegand wire effect." In essence, this device converts a zero magneto-striction material to a low magnetostriction material by introduction of physical stresses into the material. This device appears only useful in measurement of alternating magnetic fields whereas the present invention measures constant fields such as the earth's magnetic field. In particular, the device appears to be useful in measuring the frequency of AC magnetic fields.
In U.S. Pat. No. 4,687,993 to Mermelstein, an amorphous magnetostrictive ribbon is used to transfer strain to an optical fiber. This strain changes the optical length of the fiber and can be used to measure magnetic fields by measuring the phase shift of light passing through the fiber. The ribbon does not appear to exhibit or use reentrant behavior.
In U.S. Pat. No. 4,520,311 to Petr et al, in conjunction with IEEE Transactions On Magnetics, MAG-12, No. 6, November 1976, entitled "A Permalloy Current Sensor", pp 813-815 (this paper is referenced in the Petr patent), a magnetoresistive bridge is described which produces a 30 Oe wide pulse implying that the device could not have the resolution required for an electronic compass. In these references, a triangular wave drive is used as a reference and when the field to be measured crosses the level of this reference, an output is produced. Since the strongest level of the earth's magnetic field is approximately 0.68 Oe at the magnetic North Pole, Petr's device is clearly incapable of accurately measuring it. The Petr device is apparently intended for large field measurement.
The present invention provides an improved electronic compass, magnetometer and sensor therefor which is implemented by very simple circuitry and is capable of producing extremely accurate measurements without electrical alignment and without encountering many of the error sources of the prior art.